An important trend in development of semiconductor technology is scaling down of metal-oxide-semiconductor field effect transistors (MOSFETs) for improving integration level and reducing manufacturing cost. However, it is well known that short channel effects arise as the sizes of MOSFETs decrease. As the MOSFETs are scaled down, the gate also has a reduced effective length and actually controls fewer charges in the depletion region when a gate voltage is applied. Consequently, the threshold voltage of the MOSFETs drops with the reduced channel length.
In the MOSFETs, it may be desirable on the one hand that the threshold voltage of the semiconductor device is increased to suppress the short channel effects. On the other hand, it may also be desirable that the threshold voltage of the semiconductor device is decreased to reduce power consumption in a low supply voltage application, or in an application using both P-type and N-type MOSFETs. Moreover, an integrated circuit may comprise MOSFETs with different gate lengths. Although a high doping concentration in a back gate may be beneficial for the MOSFET with a small gate length for suppressing the short channel effects, it causes an excessively high threshold voltage for the MOSFET with a large gate length. It is desirable that the threshold voltages are adjusted differently for the MOSFETs with different gate lengths.
Channel doping is a known approach of tuning the threshold voltages. However, if the threshold voltage of the semiconductor device is raised by increasing a doping concentration in a channel region, the mobility of carriers will drop, which results in degradation of the device performance. Moreover, ions with a high doping concentration in the channel region may neutralize ions in source/drain regions and ions in regions which adjoin the channel region, which decreases the doping concentration in the region adjacent to the channel region and increases the resistance of the device.
Therefore, it is still desirable that the threshold voltage of the semiconductor device is adjusted in a controllable manner without increasing the doping concentration in the channel region, while the performance of the semiconductor device is not deteriorated.